The reaction of 2,4-dichloro-5-(trifluoromethyl)nitrobenzene (I) with aminomethanephosphonic acid (II) by means of NaOH in ethanol/water gives the anilinomethanephosphonic acid (III), which is condensed first with morpholine (IV) by heating at 120 C and esterified with triethyl orthoformate at 150 C yielding 5-(4-morpholinyl)-2-nitro-4-(trifluoromethyl)phenylaminomethyl phosphonic acid diethyl ester (V). The reduction of the nitro group of (V) with H2 over Pd/C in ethanol affords the corresponding amino-derivative (VI), which is cyclized with oxalic acid monoethyl ester monochloride (VII) by means of triethylamine in THF providing the quinoxaline (VIII). Finally, this compound is hydrolyzed to the free phosphonic acid with trimethylbromo silane in acetonitrile.